1. Field of the Invention
The present invention relates generally to leads for conducting electrical signals to and from the heart. More particularly, it pertains to electrode tips for delivering electrical charges to the heart, and to tips which tend to reduce power consumption from cells without reducing the effective level of each pacing pulse, and to such tips with drug delivery capability from an internal source on a lead or support element carrying the tip.
2. Background of the Invention
Leads implanted in the body for electrical cardioversion or pacing of the heart are generally known in the art. In particular, electrically transmissive leads may be implanted in or about the heart to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias or to stimulate contraction (pacing) of the heart. Electrical energy is applied to the heart via the leads to return the heart to normal rhythm. Leads have also been used to sense conditions, materials or events (generally referred to as "sense" or "sensing") in the body, such as in the atrium or ventricle of the heart and to deliver pacing pulses to the atrium or ventricle. Tachy leads generally can at least sense, pace, and deliver defibrillation shocks. Brady leads can at least perform the combination functions of pacing and sensing the heart. One of the available functions of the pacemaker or the automatic implantable cardioverter defibrillator (AICD) is to receive signals from a lead and interpret signals. In response to these signals, the pacemaker can decide to pace or not pace. The AICD can decide to pace or not pace, and shock or not shock. In response to a sensed bradycardia or tachycardia event, a pulse generator produces pacing or defibrillation pulses to correct the condition. The same lead used to sense the condition is sometimes also used in the process of delivering a corrective pulse or signal from the pulse generator of the pacemaker.
Sick sinus syndrome and symptomatic AV (atrial-ventricular) block constitute two of the major reasons for insertion of cardiac pacemakers today. Cardiac pacing may be performed by the transvenous method or by leads implanted directly onto the ventricular epicardium. Most commonly, permanent transvenous pacing is performed using a lead positioned within one or more chambers of the heart. A lead, sometimes referred to as a catheter, may be positioned in the right ventricle or in the right atrium through a subclavian vein or other vascular port, and lead terminal pins are attached to a pacemaker which is implanted subcutaneously. The lead may also be positioned in both chambers, depending on the lead, as when a lead passes through the atrium to the ventricle. Sense electrodes may be positioned within the atrium or the ventricle of the heart as appropriate for the particular condition or the choice of the medical practitioner.
Pacemaker leads represent the electrical link between the pulse generator and the heart tissue which is to be excited. These pacemaker leads include single or multiconductor coils of insulated wire having an insulating sheath. The coils provide a cylindrical envelope or tube, many times referred to as a lumen, which provides a space into which a stiffening stylet can be inserted. The conductive coil is connected to an electrode in an electrode assembly at a distal end of a pacing lead. Typically, a terminal member is molded or mounted within a flexure sleeve at the proximal end of the pacing lead and connected or joined to the proximal end of the conductive coil.
After the electrode assembly is positioned at a desired location within the heart, it is desirable to provide some method for securing the electrode assembly at that location. Mechanical fixation devices are used to firmly anchor the electrodes in the heart. One type of mechanical fixation device used is a corkscrew, or a helix electrode connector. During placement of the lead, the tip of the lead travels intravenously through veins and the heart. While traveling through the veins, the helix electrode connector at the tip of the lead may snag or attach to the side wall of the vein. Since this is highly undesirable as it may cause damage or other complications to a patient, retractable helixes are one of the optional constructions which have been provided for leads. In addition, temporary caps over the helix (such as an aqueous soluble cap, particularly a water soluble, innocuous organic material such as a sugar, starch or other biologically inert, or digestible material such as sugars, starches and the like (e.g., mannitol, sorbitol)) may be formed over the helix or tip. Preferably these materials are at least soluble or dispersible and preferably are inert or even digestible.
When using a retractable helix, the helix is extended and screwed into the heart muscle by applying a torque to the other end of the conductor without use of any further auxiliary device or with a special fixation stylet. A fixed or non-retractable helix electrode connector needs only to be positioned and secured to the heart muscle by the application of torque. Retractable helix designs employing special stylets are also available. If a soluble/dispersible cap is present on the helix, the cap must be given sufficient time to dissolve or disperse before complete securement of the helix electrode connector is attempted. A lead must be capable of being firmly secured into the wall of the cardiac tissue to prevent dislodgement therefrom, while avoiding perforation of the electrode completely through the cardiac tissue.
The pulse generator circuitry and power supply work in concert with the electrodes as a system which provides electrical pulses to the heart tissue. A low impedance electrode design may increase power delivery to the heart tissue, but at the same time, this higher energy usage results in shorter battery life. Shorter battery life is undesirable, since it increases the average number of surgical procedures to perform battery replacement for a patient.
There is a need for a body-implantable lead that has a helix for fixation to the wall of the atrium or ventricle of the heart. A separate desirable feature in body-implantable leads is for a lead having an electrode for positioning within the atrium or ventricle that allows for tissue ingrowth. Tissue ingrowth further enhances the electrical performance of the lead. The lead and electrode are further stabilized within the heart as a result of tissue ingrowth. Furthermore, there is a need for a relatively high pacing impedance electrode design which offers reasonable average voltage to the tissue. Such a high pacing impedance electrode would effectively reduce energy utilization and as a consequence, extend battery life.
It is also known to provide drugs to the patient's body before, during or after insertion of a lead or electrode. It is particularly well known to provide anti-inflammatants such as steroids, antibiotics, anti-fungal materials and the like in the region where the insertion has occurred. This is typically effected by implantation of a separate element which provides a drug (e.g., by elution from a carrier, dissolution from a slightly soluble mass, diffusion through a semi-permeable wall, slow release through a porous surface, or the like. A collar on a tip has also been reported in the literature as represented by the use of a steroid elution collar on the "Sweet Tip" Rx lead (CPI Guidant). In that embodiment, the entire drug delivery component is a collar which is fixed or slidably attached to the outside surface of the electrode or lead. This allows for the possibility of damage to the elution collar, in spite of care taken in packaging the lead.